Linear ball bearings have long been manufactured from large, unitary pieces of hardened and laboriously milled and grooved tool steel. Such prior art construction requires an expensive, hardened mass of tool steel with a machined groove throughout its interior, which undergoes undesirable dimensional changes during hardening and necessitates costly and detailed grinding to reinstate the critical dimensions of the original design.
Furthermore, the long ball bearing housings typically employed in the past have the disadvantage of requiring a relatively large number of balls involving additional expense and diminished efficiency. Other troublesome aspects of the linear ball bearings often encountered in the prior art include unwieldly, protruding adjustment mechanisms which not only constrict the device's versatility, but also fail to provide an accurate and reliable means of fitting the balls against their load. In addition, these previous high-profile and bulky linear bearings incorporate fixed, permanent race surfaces which may not be repaired or replaced. Such a limitation makes the entire prior art linear bearings useless when the race surfaces become worn or in any way damaged.
Many of the prior art linear ball bearing units are not efficient in their utilization of space. The apparent intent of their manufacturers is to reduce the loading on each ball by causing a relatively large number of balls to share the load by providing long straight races, but the result is a long, bulky unit, with uneven wear. In my opinion such long linear bearing units are wasteful of space and undesirable for reasons to be explained. A careful examination of the lengthy straight race, after the long linear bearing unit has been subjected to considerable usage, has shown me that wear actually is concentrated mainly at each end of the long straight races. In other words, the balls rolling along the middle portion of the long race usually are carrying a smaller share of the load then those rolling along near the ends. This uneven wear is caused by the fact that the opposing way, regardless of how carefully it is machined and mounted, will deflect somewhat, for it is not infinitely rigid. The machinist is seeking an absolutely dead straight way, but there is always some deviation with "hills" and "valleys". Thus, the opposing way will often have one or more concave arcuate regions. The long straight race of such lengthy prior art units travels like a geometric chord spanning across the concave arc of the opposing way, causing loading and sometimes over-loading to occur on balls near each end of such a long race. As stated above, their long length is an inefficient use of space and does not result in an appropriate sharing of the load among the balls rolling along the long race.
Furthermore, it is more difficult and expensive and requires more expertise to mount a long linear bearing unit effectively parallel to an opposing way than it is to mount a short unit. The linear ball bearing unit or the opposed way may be mounted in a machine slightly skewed so that they are not exactly in the same plane. One end of the bearing unit may be too high and the other end too low, causing excessive wear and often causing overloading of the balls at the ends of the long race. The longer bearing unit needs more sophisticated shimming mounting than a shorter one. Indicator mounting and skillful expertise is often needed with a longer bearing unit in order to meet the requisite effective parallelism between the opposing way and the long race in the long bearing unit.
In summary, the longer linear bearing unit is wasteful of space, is more difficult and expensive to mount properly, and is less tolerant of any inaccuracies and deflections of the opposing way. The longer unit does not roll over the "hills" and "valleys" of the opposing way as well as a shorter unit and wears unevenly.
Consequently, I have concluded that the maximum length of the straight race should not exceed six times the diameter of the ball bearings. Then, in spite of some deflections in the opposing way, in spite of inaccurracies in machining and mounting, all of the relatively few (six or less) balls will actually share in carrying the load, and the wear will be much more evenly distributed along the length of the short straight race. The shorter unit is more "forgiving" and keeps more balls in active loadbearing use in proportion to the total number of balls in the unit and readily avoids overloading of balls at each end of the race.